A fuel cell is an electric cell whereby a reaction energy of a gas as a feed material is converted directly to electric energy, and a hydrogen-oxygen fuel cell presents no substantial effect to the global environment since its reaction product is only water in principle. Especially, a polymer electrolyte fuel cell employing a polymer membrane as an electrolyte, can be operated at room temperature to provide a high power density, as a polymer electrolyte membrane having high ion conductivity has been developed, and thus is expected to be a prospective power source for mobile vehicles such as electric cars or for small cogeneration systems, along with an increasing social demand for an energy or global environmental problem in recent years.
In a polymer electrolyte fuel cell, a proton conductive ion exchange membrane is commonly employed as a polymer electrolyte, and an ion exchange membrane made of a perfluorocarbon polymer having sulfonic acid groups, is particularly excellent in the basic properties. In the polymer electrolyte fuel cell, gas diffusion type electrode layers are disposed on both sides of the ion exchange membrane, and power generation is carried out by supplying a gas containing hydrogen as a fuel and a gas (such as air) containing oxygen as an oxidizing agent to the anode and the cathode, respectively.
In the reduction reaction of oxygen at the cathode of the polymer electrolyte fuel cell, the reaction proceeds via hydrogen peroxide (H2O2), and it is worried that the electrolyte membrane may be deteriorated by the hydrogen peroxide or peroxide radicals to be formed in the catalyst layer. Further, to the anode, oxygen molecules will come from the cathode through the membrane, and it is conceivable that at the anode, hydrogen molecules and oxygen molecules will undergo a reaction to form radicals. Especially when a hydrocarbon membrane is used as the polymer electrolyte membrane, it is poor in the stability against radicals, which used to be a serious problem in an operation for a long period of time. For example, the first practical use of a polymer electrolyte fuel cell was when it was adopted as a power source for a Gemini space ship in U.S.A., and at that time, a membrane having a styrene/divinylbenzene polymer sulfonated, was used as an electrolyte membrane, but it had a problem in the durability over a long period of time.
As opposed to such a hydrocarbon type polymer, a perfluorocarbon polymer having sulfonic acid groups has attracted attention as a polymer excellent in the stability against radicals, and an ion exchange membrane made of such a polymer is known to be useful as an electrolyte membrane. And, in order to further increase the stability against radicals, a system having a compound with a phenolic hydroxyl group or a transition metal oxide capable of catalytically decomposing peroxide radicals incorporated to the polymer electrolyte membrane (JP-A-2001-118591) or a technique of supporting catalytic metal particles in the polymer electrolyte membrane to decompose hydrogen peroxide (JP-A-06-103992) is also disclosed. However, such a technique is a technique of incorporating a material only to the polymer electrolyte membrane, and is not one attempted to improve the catalyst layer as the source for generating hydrogen peroxide or peroxide radicals. Accordingly, although at the initial stage, the effect for improvement was observed, there was a possibility that a serious problem would result in the durability over a long period of time. Further, there was a problem that the cost tended to be high.